The present invention relates to a twin-wire former for the production of a web of paper, or of a similar fiber web, formed from a suspension of fibers wherein the suspension is fed to the twin-wire former in the form of a machine-wide jet from a head box.
Various embodiments of such twin-wire formers are known, for instance, from German Application OS 31 07 730 or U.S. Pat. No. 4,609,435.
German Application OS 31 07 730 discloses a so-called hybrid twin-wire former, i.e. it has a relatively long single-wire pre-dewatering zone. Following that first zone, a twin-wire zone commences at a forming roll which lies within the loop of the upper wire belt or, simply, upper wire. At the forming roll, the twin-wire zone is curved downwardly, around a relatively small sector of the forming roll. Herein the curving of wire zones "upward" and "downward" is described. An upward curve means that the zone is convex upward, that is toward the upper wire. A downward curve means that the zone is convex downward, that is toward the lower wire. That roll sector lies in the lower ascending quadrant of the forming roll and amounts to about 45.degree. to 60.degree. . A subsequent and adjacent upwardly curved section of the twin-wire zone is formed by a support roll which lies within the loop of the lower wire. The twin-wire zone may end, i.e. the place where the two wires separate from each other, at the support roll or at or after suction boxes which are arranged directly after or downstream of the support roll. The web formed then travels, together with the lower wire, through a "single-wire final dewatering path" which first includes further suction boxes and thereafter includes a wire suction roll for further dewatering of the web. This known twin-wire former is suitable for the production of different types of paper, but only within relatively narrow limits, because it has disadvantages. The thicker is the web of paper to be produced, i.e. the higher the specific basis weight desired, the greater is the required length of the single-wire pre-dewatering zone for a given desired speed of operation to be maintained. Furthermore, the web is formed in the single-wire pre-dewatering zone initially only in a lower layer of the fiber suspension that has been fed. The upper layer of suspension in this case remains initially liquid. As a result, it has a very great tendency to form flakes. Because these flakes can be dissolved only incompletely, if at all, in the following twin-wire zone, the resulting web of paper has a "formation" which is frequently unsatisfactory, i.e. the distribution of fibers in the finished paper web is not uniform. Despite the presence of the twin-wire zone, sometimes there is even a danger that the quality values of the finished web of paper will not be sufficiently uniform on the bottom and top, i.e. the web has a certain two-sidedness.
Another known twin-wire former with similar arrangement of a forming roll and a support roll is described in U.S. Pat. No. 4,609,435. In contrast to the previously described twin-wire former, however, this device is developed as a so-called "nip former" because there is no single-wire pre-dewatering path. Thus, the formation of the fiber web from the suspension of pulp fed from the head box takes place exclusively between the two wire belts. A slightly upwardly curved initial region of the twin-wire zone is present upstream of the forming roll. This known twin-wire former can possibly produce a web with good formation and only slight two-sidedness. But, this is true only for the production of relatively thin paper webs (i.e. on an order of magnitude of 30 to 60 g/m.sup.2) with extremely high operating speeds, as indicated in the specification of that U.S. patent.